a combining form occurring in loanwords from Greek and Latin, where it meant “eight” (octagon; octastyle), on this model, used in the formation of compound words, and in chemical terms specialized to mean “having eight atoms” (octavalent).
A Long List of Affixes: Suffixes, Prefixes, and Combining FormsSuffixes -able, -ible, -ile: (form adjs) able to, fit to, worthy, capable; apt to; subject to being ~-ed -ac: one affect with -ac, -al, -ane, -ar, -ary, -ch, -ese, -ic, -ical, -id, -ile, -ine, -ish, -ory: like, of, pertaining to; characterized by -aceae: families of plants -aceous, -ous: resemblance to a substance; full of -acy, -age, -ance, -ancy, -asm, -dom, -ence, -ency, -hood, -ism, -ity, -ment, -mony, -ness, -ry, -ship, …
Origin of octa-
< Greek okta-, combining form of oktṓ eight
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a variant of octo-
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
before vowels oct-, word-forming element meaning "eight," from Greek okta-, okt-, from PIE *okto(u) "eight" (see eight). The variant form octo- often appears in words taken from Latin, but the Greek form is said to be the more common in English.
Online Etymology Dictionary, © 2010 Douglas Harper